Mutsuki is kindhearted and gentle but starts off as shy with little self-confidence. Having constant bouts of anemia, ironically, Mutsuki feels repulsed by blood. Due to being sickly and his low self-esteem, he faces discouragement as a ghoul investigator to the point where he feels as though he is not even needed for the job. He expresses frustration with his own weakness, feeling anger at his lack of ability compared to his comrades. However, he is reliable and hard-working, always following orders in contrast to the more troublesome or lazy members of his team. Though lacking in confidence, he shows a strong desire to help others and does not hesitate to place himself at risk to protect the innocent. He eventually gains a boost in confidence and becomes much more assertive. He takes a more active role in having his teammates work together and begins to branch out to other people.He shows great respect to Sasaki and Dr. Shiba, as well as other investigators even when treated poorly by them. He is also a confidant to his superior, as he always reassures him when he is feeling down, thus showing a great fondness and admiration for Sasaki.